1. Field of the Invention
The present invention relates to a light emitting diode (LED) and a fabrication method thereof, and more particularly, to a flexible light emitting diode with preferable front light emitting efficiency and a fabrication method thereof.
2. Description of Related Art
The LED is a semiconductor element that has been widely used in light emitting devices. Generally, the LED chip is made up of III-V group compound semiconductors, such as GaP, GaAs, and GaN. The light emitting principle is to convert electrical energy into light, that is, a current is applied to the compound semiconductor, and by combining electrons with holes, the energy is converted into light so as to achieve the light emitting effect. Since LEDs have the advantages of rapid response speed (generally within about 10−9 seconds), preferable monochromaticity, small volume, low electrical power consumption, low pollution (free of mercury), high reliability, applicability for mass production processes, etc., they are widely used, such as in traffic light signals, display panels, and display interfaces of various portable electronic devices, etc.
Basically, an LED comprises a p-type III-V group compound, an n-type III-V group compound, and a light emitting layer sandwiched there-between. Further, an LED is fabricated by means of epitaxy. The light emitting efficiency of an LED element mainly depends on the internal quantum efficiency of the light emitting layer and the light extraction efficiency of the element, i.e., the external quantum efficiency.
As a light source of the flexible display, the LED encounters the following challenges. (1) LED dies belong to hard and brittle materials, so the LED dies are not flexible. (2) When an LED is applied in a backlight module, the LED lamp is combined with a large size substrate. However, the heat dissipation of the LED is poor over long time usage, so the service life of the LED is thereby shortened. (3) Since the substrate, such as GaAs substrate, used by the LED itself may absorb light, the light emitting efficiency of LED is degraded. (4) Since an LED fabricated through the current process is a bit thick, displays employing such LEDs cannot be developed towards miniaturization.
In order to solve the problems of poor heat dissipation and low light emitting efficiency of the LED, various LED structures and fabrication methods are provided.
US Patent Publication No. 2003/0085851 discloses an LED and a fabrication method thereof. Referring to FIG. 1, as for the fabrication method of LED 10, an insulating specular reflection layer 14 is formed on the bottom of an epitaxy layer 13, and a metal adhesion layer 12 is formed on the top of a silicon substrate 11 corresponding to the epitaxy layer 13. The epitaxy layer 13 comprises an n-type interface 13A and a p-type interface 13B. Next, the epitaxy layer 13 is bonded onto the top of the silicon substrate 11 through the specular reflection layer 14 and the metal adhesion layer 12 by means of hot pressing, and a temporary substrate (not shown) used for epitaxy is removed. Subsequently, an n-type ohmic contact electrode 15 and a p-type ohmic contact electrode 16 are respectively formed on the n-type interface 13A and the p-type interface 13B through coating and etching processes. Thereby, the LED 10 is fabricated.
As for LED 10 shown in FIG. 1, the light reflection efficiency is mainly improved through configuring the specular reflection layer 14, thereby improving the light emitting efficiency, and also the high heat conductivity of the silicon substrate 11 is used to improve the heat dissipation effect. However, since the n-type ohmic contact electrode 15 and the p-type ohmic contact electrode 16 are disposed on the light emitting surface of LED 10, the emission of the light may be affected, thereby the light emitting efficiency is degraded. Furthermore, since the silicon substrate 11 has a certain thickness, the fabricated LED 10 cannot meet the requirements of miniaturization.
Furthermore, U.S. Pat. No. 6,555,405 discloses a semiconductor element with a metal substrate. FIGS. 2A and 2B are cross-sectional views of the manufacturing flow of the semiconductor element with a metal substrate. First, referring to FIG. 2A, a substrate 21 is provided, and an n-type semiconductor layer 22, a light emitting layer 23, and a p-type semiconductor layer 24 are sequentially grown thereon. After the epitaxy process, a thick metal substrate 25 is then formed on the p-type semiconductor layer 24. Next, referring to FIG. 2B, the substrate 21 is removed by means of etching, polishing, etc., and the chip is reversed. And finally, a contact pad 26 is formed on the n-type semiconductor layer 22 to complete the fabrication of the LED 20.
The LED 20 shown in FIG. 2B is mainly characterized in that, the metal substrate 25 is used to replace the conventional semiconductor substrate, and the heat dissipation efficiency of the LED 20 is improved through the high heat conductivity and high electrical conductivity of the metal substrate 25, thereby improving the light output efficiency. However, since the contact pad 26 is disposed on the light emitting surface of the LED 20, the light emitting efficiency of the LED 20 is degraded. Additionally, the thickness of the current LED is about 120 μm to 200 μm, but it can still become thinner.
Although the above-mentioned two LEDs with different forms have partially solved the problems of heat dissipation and light emitting efficiency through different methods, the structures of the two LEDs still cannot overcome the problems of flexibility and miniaturization. Therefore, how to improve the efficiencies of heat dissipation and light emitting of the LED by changing the structure of the LED and meanwhile achieving the purposes of flexibility and miniaturization is a vital issue to be solved.